Reshaping of the cornea for refractive vision correction has been the object of various procedures, some of which have only been recently developed. In one well known procedure, namely, radial keratectomy (RK), the cornea is incised with radial cuts to flatten the shape of the anterior surface of the cornea in order to correct for myopia. This is a surgical procedure requiring a high degree of skill and judgment for effective and safe implementation.
Additionally, even when such procedure is carried out properly, myopia-corrective flattening may cause instabilities, such as a gradual progression to hyperopia over time.
Another recently developed system uses an chimer laser to remove corneal tissue by photo-thermal ablation rather than cutting. In the latest version of this system, a sequence of incident laser pulses with energy focused to a small spot moving from point to point gradually removes tissue from the anterior surface of the cornea. The local extent of tissue removal depends on the number of laser pulses at the position on the cornea and results in a new shape for the ablated surface. An eye tracker is used in some versions to compensate for eye motion during the lengthy scan period, e.g., tens of seconds. Laser spot scanners utilize a bell-shaped laser energy distribution having a half-power diameter of about 2 mm. It is likely that smaller laser spots could be achieved, but stability of the eye or accuracy of the eye tracker may limit the useful resolution. The pulsed delivery of laser energy in spots and arbitrary spot positioning allows overlap during scanning for smoothing. The equivalent spot density can be high. Nevertheless, the achievable resolution or shaping detail is limited by the spot size, since overlapped spots are not independent. Also, the spatial frequency transfer function for patterning effects the accuracy of the laser spot scanners. Based on the 2 mm spot size, the distribution is probably flat out to a cutoff spatial frequency of about 0.25 cycles/mm. Over a 6 mm ablation zone, that implies only 1.5 cycles of shaping. It seems barely enough for myopic correction. For hyperopic correction, a 9 mm zone would be required. It may not be enough for achieving super acute vision where finer features need to be resolved.
This use of laser pulses for shaping the cornea, known as photo-refractive keratectomy (PRK) is generally safe and effective. However, there are several drawbacks to this method, including the high cost of the equipment required for the PRK procedure. Another drawback is the relatively high residual error factor (or lack of emmetropia), often on the order of .+-.1.0 diopter more, as compared to a typical error of less than .+-.0.25 diopter for spectacles or contact lenses. In addition, laser ablation results in a rough corneal surface. Furthermore, there are long term effects relating to the physiology of the cornea and its interaction with the laser during ablation, which may result in subsequent gradual reversal of the correction and/or complications due to wound healing and/or potential carcinogenic effects. Other common side effects of PRK include haze, night-time glare and reduced best-corrected visual acuity.
The cornea comprises a thin protective epithelium layer on top of the Bowman's membrane or layer, which in turn covers the major corneal stroma. While the epithelium is regenerative, the Bowman's membrane is not. With ablative corneal tissue removal procedures such as PRK, the epithelium and Bowman's membrane are removed together with a portion of the stroma. Subsequently, the epithelium regenerates on the exposed outer surface of the cornea directly on the stroma because the Bowman's layer is not regenerated. However, direct regrowth of the epithelium on the stroma can cause an undesirable corneal haze which gradually dissipates over time.
Both the RK and PRK methods described above have inherent instabilities and error factors which make them generally unsuitable for correction of myopia of more than -9 diopters. A surgical procedure known as Automated Lamellar Keratoplasty (ALK) preserves the Bowman membrane and has been used for corrections of up to -20 diopters. In this procedure, in a first surgical step, a blade micro-keratome is used to remove a uniform thickness button or lenticule of corneal tissue which contains a portion of the epithelium layer, the Bowman's membrane (intact) and a portion of the stroma. The button or lenticule preferably remains "hinged" at one point to the cornea. The hinged lenticule is then moved out of the way and the stromal bed is surgically reshaped with the micro-keratome by removal of a second unhinged lenticule to produce the required refraction correction. Then, the hinged lenticule is replaced on the stromal bed, providing good adherence and healing of the stroma-stroma interface, preserving the Bowman's membrane, and leaving the cornea substantially clear. It appears that the stroma-stroma healing of the ALK procedure reduces, if not eliminates, wound healing instabilities, making this procedure suitable for large refractive corrections.
However, despite the advantage of retention of vision clarity and healing stability, the ALK procedure is not favored because it is complex and expensive, requires high surgical skills and, depending on the surgeon's skill, is usually inaccurate and may cause irregular astigmatism. Some of these problems may be attributed to the viscous and generally unsupported nature of the cornea, which may be enhanced by reflexive movements of the patient, making the use of a scalpel or even a micro-keratome difficult and inaccurate.
In view of the above, currently the most favored approach to refraction correction is to produce a hinged flap with a blade micro-keratome and then to reshape the exposed stromal bed using PRK as described above. This procedure, commonly referred to as LASIK, is less safe than conventional PRK and is used primarily because of reduced short-term inconveniences, such as pain and delay in return of visual acuity. The long term effects of LASIK are similar to those of PRK.
Cleaving off a lenticule having a predetermined shape using a microjet beam is also known in the art. Such a procedure is described in U.S. Pat. No. 5,556,406 to Gordon et al., the entire disclosure of which is incorporated herein by reference. In practice, a number of different procedures using a microjet beam have been applied for refraction correction.
In a procedure known as the HRK1, by Medjet Inc. (Edison, N.J.), a lenticule having a desired shaped is removed by a microjet beam. After this removal, epithelium growth on the remaining stromal bed may change the optical properties of the cornea causing inaccuracies in the refraction correction. This phenomenon is similar to that described above with reference to PRK. Another procedure using a microjet beam, known as HRK2, is similar to the two-step ALK technique described above. In a first step, a microjet beam cut is used to form a hinged flap in the cornea. The flap is then moved to the side and a second cut is made with the microjet beam, removing a lenticule of a predetermined shape for refractive correction. Finally, the flap is replaced in its original position. The results are similar to those of the ALK technique, but the use of a water jet beam is safer and more accurate. This technique is described in U.S. Pat. No. 5,556,406 to Gordon et al.
By investigating the interaction of a fluid microjet beam with the cornea, the present inventors have discovered that a single lamellar cut in the cornea can be used to remove inner corneal tissue under a parallel flap. When the flap is placed back on the cutting site, the resultant corneal surface is flattened compared to the original surface topography.
In a procedure known as HRK3 by Medjet Inc., shaping of the cornea by erosion and cutting a hinged flap are preformed simultaneously. According to experimental results, a surface cut by fluid microjet cannot be distinguished, under microscopic examination, from a surface cleaved by a micro-keratome. Shaped erosion removal of tissue is also possible under certain scan conditions. Experimental results also indicate that HRK tissue removal can result in a spherical surface. The thickness of removed tissue is less than or greater than the microjet beam diameter, as required. However, based on experimental results, there seems to be a practical limit to the thickness of tissue that may be removed by a single beam scan and, thus, there is a limit to the refractive change that may be achieved by this method. In general, erosion tissue removal can be increased by reducing the scanning speed of the microjet beam; however, substantial slowing of the scanning speed results in poor or even unacceptable surface quality. This technique is described in U.S. patent application Ser. No. 08/955,645, filed Oct. 22, 1997, the entirety of which is incorporated by reference. To achieve greater refraction correction by erosion shaping, a multi-scan technique has been used, wherein a high accuracy scanning robot performs multiple scans in the same plane for additional tissue removal by erosion. In this technique, greater tissue removal can be achieved by cutting and, thus, greater diopter correction. However, multiple scanning of the microjet beam is similar to slow scanning of the beam and may therefore result in poor surface quality.
Therefore, the rapid evolution of refractive surgery based on the LASIK procedure and the increasing interest in the potential of a surgical approach to achieve super acute vision has created an interest in a surgical procedure which will allow accurate and high resolution custom tissue removal. Improved refraction correction results compared to the surgical procedures described above are needed.